[Xen-devel] Re: [PATCH] x86: hold mm->page_table_lock while doing vmalloc_sync

[Larry Woodman <lwoodman@xxxxxxxxxx>]

On 02/02/2011 09:48 PM, Andrea Arcangeli wrote:
> Hello,
>
> Larry (CC'ed) found a problem with the patch in subject. When
> USE_SPLIT_PTLOCKS is not defined (NR_CPUS == 2) it will deadlock in
> ptep_clear_flush_notify in rmap.c because it's sending IPIs with the
> page_table_lock already held, and the other CPUs now spins on the
> page_table_lock with irq disabled, so the IPI never runs. With
> CONFIG_TRANSPARENT_HUGEPAGE=y this deadlocks happens even with
> USE_SPLIT_PTLOCKS defined so it become visible but it needs to be
> fixed regardless (for NR_CPUS == 2).
>
> I'd like to understand why the pgd_lock needs irq disabled, it sounds
> too easy that I can just remove the _irqsave, doesn't it?
>
> A pgd_free comment says it can run from irq. page_table_lock having to
> be taken there is for Xen only, but other archs also uses
> spin_lock_irqsave(pgd_lock) so I guess it's either common code, or
> it's superfluous and not another Xen special requirement.
>
> If we could remove that _irqsave like below it'd solve it... But
> clearly something must be taking the pgd_lock from irq. (using a
> rwlock would also be possible as long as nobody takes it in write mode
> during irq, but if it's pgd_free that really runs in irq, that would
> need the write_lock so it wouldn't be a solution).
>
> I'm trying this fix and the VM_BUG_ON never triggered yet.
>
> In short: who takes the pgd_lock from an irq? (__mmdrop shouldn't but
> maybe I'm overlooking some aio bit?)
This is the specifics:

The problem is with THP.  The page reclaim code calls page_referenced_one()
which takes the mm->page_table_lock on one CPU before sending an IPI to other
CPU(s):

On CPU1 we take the mm->page_table_lock, send IPIs and wait for a response:
page_referenced_one(...)
        if (unlikely(PageTransHuge(page))) {
                pmd_t *pmd;

                spin_lock(&mm->page_table_lock);
                pmd = page_check_address_pmd(page, mm, address,
                                             PAGE_CHECK_ADDRESS_PMD_FLAG);
                if (pmd&&  !pmd_trans_splitting(*pmd)&&
                    pmdp_clear_flush_young_notify(vma, address, pmd))
                        referenced++;
                spin_unlock(&mm->page_table_lock);
        } else {


CPU2 can race in vmalloc_sync_all() because it disables interrupt(preventing a
response to the IPI from CPU1) and takes the pgd_lock then spins in the
mm->page_table_lock which is already held on CPU1.

                spin_lock_irqsave(&pgd_lock, flags);
                list_for_each_entry(page,&pgd_list, lru) {
                        pgd_t *pgd;
                        spinlock_t *pgt_lock;

                        pgd = (pgd_t *)page_address(page) + pgd_index(address);

                        pgt_lock =&pgd_page_get_mm(page)->page_table_lock;
                        spin_lock(pgt_lock);


At this point the system is deadlocked.  The pmdp_clear_flush_young_notify
needs to do its PDG business with the page_table_lock held then release that
lock before sending the IPIs to the other CPUs.



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